Photochemically generated hydroxyl radicals (.multidot.OH) or photochemically activated ozone (O.sub.3) are frequently used as oxidising agents due to their high oxidation potentials.
Their high oxidation potentials can, however, be a drawback in that it may be difficult to target oxidation by these agents only to a particular desired chemical compound. The products of the initial oxidation may themselves be further oxidised by these agents, thus failing to provide the desired selectivity or, in some cases, leading to the production of undesirable or hazardous by-products.
One example where oxidation by .multidot.OH or O.sub.3 is undesirable is in the oxidative degradation of hydrazines which may occur as pollutants in waste waters or contaminated groundwaters. Thus, for example, if 1,1-dimethylhydrazine (or unsymmetrical dimethylhydrazine, UDMH) is oxidised by .multidot.OH or O.sub.3, N-nitrosodimethylamine (NDMA) is produced.
NDMA is carcinogenic and has a much lower discharge requirement than UDMH. Thus if .multidot.OH or O.sub.3 is employed as oxidant to treat waste contaminated with UDMH, a long and much more expensive treatment is required to remove the NDMA formed as a byproduct. It may be difficult to achieve the low parts per trillion (ppt) discharge requirements of NDMA even with extensive treatment by .multidot.OH or O.sub.3.
An alternative oxidant, other than .multidot.OH or O.sub.3, which does not add an oxygen molecule to the contaminant, is required to treat hydrazine waste streams. Fochtman et al. (U.S. Pat. No. 4,402,836) have disclosed a method of treatment of hydrazine-containing waste waters involving the addition of chlorine gas to the waste water followed by the application of UV light. This method has several drawbacks. Chlorine is a corrosive, toxic gas, requiring special handling which adds to the cost and inconvenience of the process. Chlorine treatment can also produce toxic chlorinated byproducts if sufficient treatment time is not allowed. As well, when high levels of hydrazines must be treated, large quantities of chlorine gas are required and result in very high concentrations of residual chloride ions (Cl.sup.-) building up in the water.
Meiners et al. (U.S. Pat. No. 3,649,493) have disclosed a method for oxidizing organic contaminants by addition of a hypohalous acid and UV light. This method is safer than treatment with chlorine gas but it has several drawbacks if used to treat hydrazine-containing waste waters. Photolysis of hypohalous acids is known to produce hydroxyl radicals (.multidot.OH) and hence results in production of NDMA when UDMH is treated. As well, the treatment of high concentrations of hydrazines results in a buildup of high levels of halogen ions such as chloride ion (Cl.sup.-), which may lead to corrosion problems and may also violate discharge requirements.
Iodine (I.sub.2) is known to oxidize hydrazine and is used as an analytical method for hydrazine determination (Skoog, D. A. and West, D. M. in "Fundamentals of Analytical Chemistry:" 1982, CBS College Publishing, p 374-378). Iodine has, however, a low solubility in water making it impractical for waste water treatment. In addition, iodide ion (I.sup.-) is a byproduct of the oxidation so that non-photochemical oxidation using I.sub.2 results in the build up of I.sup.-.
The ultraviolet photolysis of iodide ions produces iodine atoms and hydrated electrons. This means of generation of hydrated electrons has been harnessed in a reductive degradation process for certain contaminants (Bolton and Cater, U.S. patent application Ser. No. 7/801,808). The possibility of photooxidation by the iodine atoms produced was not explored.
There remains a need for a convenient oxidative process for degradation of hydrazines without the formation of toxic byproducts or the build up of halogen ions.
There also remains a need for an oxidising process without the poor selectivity of oxidation by .multidot.OH or O.sub.3.